Studies have been devoted to determining how adenylate cyclase activity might be regulated in fibroblasts growing in culture. The cyclase from Kirsten and Moloney sarcoma virus transformed NRK cells has lost its responsiveness to activation by hormones. Binding studies indicate that the beta-receptor for catecholamines is present on these transformed cells, although its affinity for agonists may be somewhat decreased relative to the receptor from control NRK cells. Work is in progress to determine if the mechanism involved in coupling the occupied receptor with the catalytic subunit of the cyclase is defective. Cholera toxin activation of adenylate cyclase appears to require the presence of a plasma membrane-associated reconstituting activity (RA), along with GTP, ATP, and NAD+. Choleragencatalyzed ADP ribosylation of plasma membrane substrate proteins also requires the presence of RA. These observations indicate that an endogenous membrane-associated factor, along with GTP, may be involved in modulating the ability of choleragen to activate adenylate cyclase. Gonadotropin treatment of CHO cells increases the activity of several enzymes involved in steroidogenesis, indicating that CHO cells in culture might be used to study some of the actions of these hormones.